embedded with the iridium layer from three widely separated sites
Denmark, Spain, and New Zealand.
Wolbach's discovery stemmed from the curiosity of her research
adviser, cosmochemist Edward Anders, about what kind of extrater
restrial object had struck the planet at the K-T boundary. A meteorite
from the asteroid belt? A comet? He suggested that Wolbach attempt
to isolate carbon in the iridium layer. Carbon would have trapped cer
tain rare gases brought in by the impacting object. The isotopes of
those gases might provide chemical signatures to identify the intruder
from space.
To her surprise Wolbach has found an enormous enrichment of soot.
"To get the amount of soot we find," she says, "as much as 90 per
cent of the world's forests must have burned."
Granted the impact of a ten-kilometer body would be equivalent to
10,000 times the power of all the world's nuclear weapons, how could
fire spread so disastrously across the globe?
"Even if it hit in the ocean, the impact would have created a crater
300 kilometers across," says Anders. "A huge plume would have
pushed the atmosphere aside. The fireball would have had a radius of
several thousand kilometers. Winds of hundreds of kilometers an hour
would have swept the planet for hours, drying trees like a giant hair
dryer. Two-thousand-degree rock vapor would have spread rapidly. It
would have condensed to white-hot grains that could have started ad
ditional fires."
In addition, lightning discharges like those in a volcanic eruption
could have ignited windswept fires on all landmasses that marched far
faster than those at Yellowstone.
UCH DOOMSDAY SCENARIOS strain our belief. And many sci
entists refuse to accept that such catastrophes have caused
the great dyings.
"We don't need an impact," I have heard over and over
from paleontologists. "We can explain mass extinctions
with earthly causes."
And so they can. Falling sea levels. Ice ages. Collisions of conti
nents. Volcanism. Climate changes. Altered ocean chemistry. The
potential mechanisms for mass death are many.
No matter what causes them, mass extinctions do occur. They force
a new perspective on the history of life.
"Mass extinctions change the rules of evolution," explains David
Jablonski of the University of Chicago, one of the leading extinction
theorists. "When one strikes, it's not necessarily the most fit that sur
vive; often it's the most fortunate.
"When their environment is disrupted, groups that had been healthy
can suddenly find themselves at a disadvantage. Other species that had
been barely hanging on squeak through and inherit the earth.
"The best example is mammals. Dinosaurs and mammals originated
within ten million years of each other about 220 million years ago. But
for 140 million years dinosaurs ruled, while mammals stayed small and
scrambled around hiding out in the underbrush. Mammals all basically
looked alike -squirrelly or shrewish and no bigger than a badger
until the dinosaurs disappeared. Then they took off. Within ten million
years there were mammals of all shapes and life-styles: whales and
bats, carnivores and grazers. Mammals just couldn't do anything
interesting until the dinosaurs were out of the way."
CLUE to a killer's identity
may remain in rugged
central Quebec, where
the Manicouagan crater
was apparently blasted by
a giant meteorite some
210 million years ago.
That roughly coincides
with a mass extinction of
marine species at the end
of the Triassic period.
Sky-darkening dust from
the impact may have
played a role.
Extinctions
673